Page 45 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN006C-254 June 28, 2001 19:52

Food Colors 117

side reaction, known as the Strecker degradation, occurs oxidized to the ferric state for the blackish complex to ap-
between α-amino acids and dicarbonyl compounds and pear. The stem end, which contains much less citric acid
leads to the formation of aldehydes possessing one less than the rest of the tuber, displays the deepest darkening.
carbon atom than the corresponding initial amino acids. Canned or pickled cauliflower may turn dark due to the
The newly formed aldehydes are responsible for the pleas- interaction of polyphenols in the tissue with iron from
ing flavors. external sources.
A significant consequence of the Maillard reaction is As already indicated, nonenzymatic browning is de-
the loss of the nutritional value of the amino acid involved sirable in certain instances and undesirable in others. The
in the reaction. If the participating amino acid is essential, availabilityofreactantsandthetypeofconditions(temper-
and especially if it is a limiting one, as is lysine in most ature, pH, moisture) will determine the extent of brown-
cereal grains, the Maillard reaction can seriously lower ing. A chemical preservative often used to inhibit nonen-
the nutritive value of the food. Toasting, for example, may zymatic (and enzymatic) browning is sulfur dioxide. An
reduce to one-half the protein efficiency ratio of bread. obvious way to prevent metal-polyphenol browning is to
eliminate contact between susceptible tissues and reac-
2. Caramelization tive metals and use inoffensive equipment (stainless steel,
glass-lined tanks, etc.)
This reaction leads to brown products when sugars are
heateddryorinsolution.Certainconditionsofcarameliza- IV. COLOR ADDITIVES IN FOODS
tion favor the formation of flavor compounds as well. The
chemical transformations involved in caramelization are
Colored substances are added to foods to modify the ap-
complex and poorly understood. They include dehydra-
pearnce of insufficiently colored or discolored foods and
tion, fragmentation, and polymerization. On the heating of
to create new foods. The following criteria must be met if
pentoses, furfural is formed which polymerizes to brown
a food color (colorant) is to be used; (1) it must be safe
products. Heating hexoses results in hydroxymethylfur-
at the level and under the conditions of use; (2) it must
fural, which polymerizes similarly. The large quantities
be stable in the products in which is added; (3) it must
of industrial caramel color that are added to beverages
not impart any offensive property (flavor, texture) to the
(cola drinks), baked goods, and confections are made by
product; (4) it must be easy to apply; (5) it must have a
heating high-conversion corn syrups in the presence of
high tinctorial power; and (6) it must not be too costly.
catalysts (acids, alkalis, salts).
There are two classes of color additives, those that must
be certified and those that are exempt from certification.
3. Ascorbic Acid Browning Both are strictly controlled in the United States by regula-
tory statutes (Food Color Additives Amendments), but an
When ascorbic acid is heated in the presence of acids,
official certificate is required for each commercial batch
furfural is formed. The latter, either by itself or after
of color of the first group, while no such certificate is
reacting with amino compounds, polymerizes to brown
necessary for the second group. For certification the man-
products. Citrus juices, especially their concentrates, de-
ufacturer must submit a sample of the batch to the Food
velop browning, which has been attributed to ascorbic acid
& Drug Administration for chemical analysis. The results
degradation.
of the analysis are compared with the specifications for
certified colors published in the Code of Federal Regula-
4. Metal–Polyphenol Browning
tions. If the compliance is complete, a certificate is issued
Polyphenolic compounds form complexes with certain for that particular batch of color.
metals. The polyphenols of fruits and vegetables most When a new color is to be introduced, the petitioner is
commonly chelate iron. The resulting iron complexes expected to provide data proving that the new additive is
are bluish black pigments. Cutting apples with a non- safeandeffective.Thesafetytestsareelaborateandexpen-
stainless-steel knife results in darkening of both the blade sive. They include chronic toxicity feeding tests with two
and the cut surface of the apple. This darkening is inde- animal species over several generations. The effectiveness
pendent of the enzymatic browning that might develop as tests include long-term color stability experiments in the
a result of cutting. Wine makers avoid contact between the foods to which the color is to be added.
wineandironimplementsbecauseoftheblackiron–tannin The food color additives subject to certification are
precipitate that forms on such contact. Certain varieties of listed in Table IV. The initials FD&C stand for the Food,
potatoes tend more than others to darken after cooking. Drug & Cosmetic Act under which these additives are reg-
This darkening is attributed to a complex between iron ulated. The food color additives exempt from certification
and chlorogenic acid. The iron of the tissue must first be are listed in Table V. Generally, only synthetic organic